The present invention relates generally to switched communications systems, and more specifically to an ATM (asynchronous transfer mode) switching system for ISDN (integrated services digital network) systems.
It is known that circuit-switched networks are provided with spare links or channels which are physically preassigned as alternate routes to all operating links or channels. In the event of a failure, connections are switched from a normal link or channel to the preassigned alternate route. With packet-switching networks such as broadband ISDN systems, on the other hand, packets of fixed length, or `ATM cells` are transported on a `virtual` path that is established by successive switching nodes in response to a virtual path identifier (VPI) contained in the cells. Since the overall transmission capacity of a virtual path is determined by the transmission capacities of its constituent links, virtual paths must be treated as indivisible units for determining an alternate route. Therefore, the alternate routing technique of the circuit-switched network cannot apply to the packet-switched network.